π-Conjugated oligomeric and polymeric semiconductors have been the focus of intense research over the past three decades as potential alternatives to inorganic semiconductors for low-cost electronic components, such as organic thin-film transistors (OTFTs), light-emitting diodes (OLEDs), and photovoltaics. See, e.g., Dimitralopoulos, C. D. et al. Adv. Mater., 14: 99-117 (2002); Horowitz, G. et al., Adv. Mater., 10:365-377 (1998); Katz, H. E., Chem. Mater., 16: 4748-4756 (2004); Sirringhaus, H. et al., Science, 280: 1741-1744 (1998); Bernius, M. et al., Thin Solid Films, 363: 55-57 (2000); Kraft, A. et al., Angew. Chem., Intl. Ed. Engl., 37: 402-428 (1998); Kulkarni, A. P. et al., Chem. Mater., 16: 4556-4573 (2004); and Alam, M. M. et al., Chem. Mater., 16: 4647-4656 (2004). OTFTs can be used in low-performance memory elements, sensors, and as drive devices for active-matrix displays. See, e.g., Huitema, H. E. A. et al., Adv. Mater., 14: 1201-1204 (2002); Kitamura, M. et al., Jpn. J. Appl. Phys., Part 1, 42: 2483-2487 (2003); and Mach, P. et al., Appl. Phys. Lett., 78: 3592-3594 (2001). OLEDs are envisioned as cheap, energy-efficient alternatives to liquid crystal displays, and flat-panel displays based on OLEDs are emerging in commercial portable electronic devices and in novel textiles. Organic semiconductors enable vapor phase or solution fabrication of low-cost, large-area, light-weight electronic devices, and are compatible with plastic substrates for flexible, conformable, and wearable electronics.
Among the organic semiconductor classes used for OTFTs, (oligo, poly)-thiophenes have been among the most extensively investigated. The hole transporting properties of α-sexithiophene (α-6T) was first reported in 1988. See, e.g., Fichou, D. et al., Chemtronics, 3: 176-178 (1988). One year later, p-type OTFT devices fabricated from thermally evaporated α-6T thin films were reported. See, e.g., Horowitz, G. et al., Solid State Commun., 72: 381-384 (1989); and Horowitz, G. et al., Appl. Phys. Lett., 57: 2013-2015 (1990). The highest mobilities obtained in OTFT devices using vapor-deposited thin films currently approach those measured for α-6T single crystals (t=0.16 cm2/Vs). See, e.g., Horowitz, G. et al., Euro. Phys. J. Appl. Phys., 1: 361-367 (1998). To fully realize organic electronics via complementary circuits, high-performance electron-transporting (n-type) oligothiophenes have also been developed. See, Facchetti, A. et al., Chem. Mater., 16: 4715-4727 (2004); Facchetti, A. et al., J. Am. Chem. Soc., 126: 13859-13874 (2004); Facchetti, A. et al., J. Adv. Mater., 17: 1705-1725 (2005); Facchetti, A. et al., J. Am. Chem. Soc., 126: 13480-13501 (2004); Facchetti, A. et al., Angew. Chem., Intl. Ed. Engl., 42: 3900-3903 (2003); Jones, B. A. et al., Angew. Chem., Intl. Ed. Engl., 43: 6363-6366 (2004); Yoon, M. H. et al., J. Am. Chem. Soc., 127: 1348-1349 (2005); and Yoon, M. H. et al., J. Am. Chem. Soc., 128: 5792-5801 (2006). However, OTFT devices based on the oligothiophenes often exhibit significantly lower mobilities when the films are grown from solution, presumably reflecting difficulties in creating high levels of structural ordering from solution. Therefore, the intrinsic inefficiency of alternative vacuum vapor phase film growth processes renders the oligothiophenes less appealing as active channel materials in OTFTs.
In order to take full advantage of the cost efficiencies of solution processing methods such as spin-coating, stamping, or inkjet printing, polymeric organic semiconductors would seem to be the materials of choice. Among polythiophenes, soluble regioregular polythiophenes, such as poly(3-hexylthiophene) (P3HT) and variants, see, e.g., Bao, Z. et al., Appl. Phys. Lett., 69: 4108-4110 (1996); Bao, Z. et al., Chem. Mater., 11: 2607-2612 (1999); Merlo, J. A. et al., J. Polym. Sci., Part B: Polym. Phys., 41: 2674-2680 (2003); Sirringhaus, H. et al., Synth. Mat., 202: 857-860 (1999); and Sirringhaus, H. et al., Nature, 401: 685-688 (1999), are the most commonly used in OTFT applications due to their high charge-carrier mobilities and chemical availability. Despite recent advances, one of the major drawbacks of commonly used polythiophenes is their poor stability in air. This shortcoming has been particularly acute when these materials are used as the active layers in OTFTs. Doping of polythiophenes by reaction with ambient O2 often results in large off-currents and thus lower current on/off ratios (Ion/Ioff), as well positive shifts in the threshold voltage for the transistors fabricated from these materials. See, e.g., Meijer, E. J. et al., J. Appl. Phys., 93: 4831-4835 (2003). Therefore, precautions must be taken during materials synthesis and device fabrication to exclude O2. These constraints render polythiophene-based OTFTs less attractive as cheap alternatives to silicon-based chips, and there is a great need to develop semiconducting polymers with both high carrier mobility and enhanced air stability.
Silicon substituents have long been known to stabilize adjacent carbanions because of their strongly electron-withdrawing character. See, e.g., Wetzel, D. M. et al., J. Am. Chem. Soc., 110: 8333-8336 (1988). Among the various silicon-containing π-conjugated systems, silole (sila-2,4-cyclopentadiene) polymers have recently attracted broad attention as novel conjugated systems in which the Si—C σ*-orbital effectively interacts with the π*-orbital of the butadiene fragment, leading to a low-lying LUMO and relatively small band gaps. See, e.g., Risko, C. et al., J. Chem. Phys., 121: 9031-9038 (2004); Yamaguchi, S. et al., J. Chem. Soc., Dalton Trans., 3693-3702 (1998); Zhan, X. W. et al., J. Am. Chem. Soc., 127: 9021-9029 (2005); and Yamaguchi, S. et al., Bull. chem. Soc., Jpn., 69: 2327-2334 (1996). Additionally, the introduction of silicon also results in stabilization of the silole HOMO level, compared to their carbon counterparts, which should, a priori, help to improve the ambient stability of silole-containing polymers in OFET devices. To date, however, the use of silole derivatives has been limited to electron-transporting materials in OLEDs and solar cells. See, e.g., Chan, K. L. et al., J. Am. Chem. Soc., 127: 7662-7663 (2005); Chen, H. Y. et al., Appl. Phys. Lett., 81: 574-576 (2002); Chen, J. W. et al., Chem. Mater., 15: 1535-1546 (2003); Kim, W. et al., Chem. Mater., 16: 4681-4686 (2004); Liu, M. S. et al., Chem. Mater., 15: 3496-3500 (2003); Luo, J. D. et al., Chem. Commun., 1740-1741 (2001); Murata, H. et al., Appl. Phys. Lett., 80: 189-191 (2002); Tamao, K. et al., Chem. Commun., 1873-1874 (1996); Tamao, K. et al, J. Am. Chem. Soc., 118: 11974-11975 (1996); Ohshita, J. et al., Organometallics, 18: 1453-1459 (1999); and Mi, B. X. et al, Chem. Commun., 3583-3585 (2005). Not until very recently have silole-containing polymers been used as the active layers in OTFTs. See, e.g., Ohshita, J. et al, Chem. Lett., 33: 892-893 (2004); Wang, Y. et al, Macromol. Chem. Phys., 206: 2190-2198 (2005); and Wang, F. et al, Macromolecules, 38: 2253-2260 (2005). Nevertheless, the performance of the reported materials is generally poor in regard to both carrier mobility and Ion/Ioff, probably because steric hindrance between large substituents at the 3 and 4 positions of the thiophene interferes with the close π-π stacking requisite for efficient charge transport.